Air Traffic Control (ATC) functions to control flights in a controlled airspace, i.e., an airspace that is continuously controlled throughout a flight so that a specific separation between aircraft, both vertically and horizontally, is achieved. ATC is provided by either an Area Control Center (ACC) when the aircraft flies within a Flight Information Region (FIR), or by the Approach (APP) or Tower (TWR) units of a controlled airport upon takeoff or landing. In order for a flight to be controlled, voice communications between the pilot of the aircraft and the flight controller have to be continuously available and the aircraft has to be monitored via radar.
Very High Frequency (VHF) communications are achieved by a large number of earth-based VHF transceiver stations. The interconnection of the VHF transceivers with the ATC center is implemented using a ground communication networking infrastructure. At the ATC center, a radio PBX (e.g., the Voice Communication System, or VCS) handles the signals to and from the VHF transceiver sites and routes voice communications to flight controllers. The same PBX handles the interconnection with the airports, which have their own flight controllers.
In areas with mountainous terrains, it is extremely difficult, if not impossible, to provide the necessary communication infrastructure so that the remote VHF transceiver station can relay voice to and from the ATC center. This poses great threats to flight safety, often forcing the authorities to close parts of the airspace.
The use of satellite communications offers a viable alternative to VHF transceiver stations where it is not possible to use ground means to implement the communication infrastructure. However, though satellite communications offer exceptional geographical coverage, they also induce a large amount of transmission delay. For example, for geostationary trajectories, transmission delay is equal to approximately 260 milliseconds from one point to another, if a single hop is assumed. This delay poses important problems to aeronautical communications. For example, since the reaction time of pilots and controllers to difficult situations depends on the delay of the channel that they use to communicate, the minimum allowed separation depends on that delay. In addition, radio call control is adversely affected, because if ground communication means such as leased lines are used, the delay is in the order of some microseconds.